1. Field of the Invention
The present invention relates to a sealed rectangular battery including an explosion-proof valve in a square shaped battery case.
2. Description of the Related Art
Recently, compact and light electronic apparatuses such as portable phones or personal digital assistances are becoming widespread. A compact and light battery having high-capacity is being used as a power source. As the electronic apparatus is becoming more sophisticated, the battery is being required to have higher capacity. Particularly, a primary battery using lithium metal as an active material, and a secondary battery for performing charge and discharge using lithium ion is being widely used because of its light weight and high-capacity, and a development in higher capacity thereof is also active.
In such battery, when an excessive electrical load is applied or an excessive thermal load is applied, a short-circuit state occurs within the battery thus generating gas, and an internal pressure of the battery rises abnormally. Even when the battery is over charged, gas is generated within the battery due to decomposition of an electrolytic solution, and the internal pressure of the battery rises abnormally.
When the battery can no longer tolerate the abnormal rise of the internal pressure, the battery case explodes and the contents fly out in all directions. Thus, the battery case is equipped with an explosion-proof valve for releasing internal pressure of the battery when the internal pressure exceeds a predetermined internal pressure thus preventing the explosion of the battery.
The explosion-proof valve is disclosed in for example, Japanese Laid-Open Patent Publication No. 10-261391 (Paragraphs 0017-0019, FIG. 3), Japanese Laid-Open Patent Publication No. 11-185714 (Paragraphs 0019, 0026, FIGS. 2-4), and Japanese Laid-Open Patent Publication No. 2001-143664 (Paragraphs 0019-0022, FIGS. 1-6). In such publications, a groove is formed on a lid and the like of the battery case, and when the internal pressure of the battery becomes equal to or greater than a predetermined value, a thin wall formed by the groove rips thus releasing the internal pressure of the battery. Another document, for example, Japanese Laid-Open Patent Publication No. 2001-256944 (Paragraphs 0021-0028, FIGS. 1-12) discloses an explosion-proof valve in which a circular or an elliptical thin part is formed on the lid of the battery and when the internal pressure of the battery becomes equal to or greater than the predetermined value, the thin part rips thus releasing the internal pressure of the battery.
With regards to the explosion-proof valve disclosed in Japanese Laid-Open Patent Publication No. 10-261391, Japanese Laid-Open Patent Publication No. 11-185714, and Japanese Laid-Open Patent Publication No. 2001-143664, when the battery case expands and deforms due to the abnormal rise of the internal pressure of the battery, the groove is pulled in the direction of the width of the groove and is ripped, and thus can not be sufficiently ripped unless the dimension of the entire length of the groove is long. Therefore, a large surface area is needed to form the groove, and is thus difficult to apply to a small battery.
In this case, when the wall thickness of the thin wall of the groove is thinned, the technique disclosed in Japanese Laid-Open Patent Publication No. 10-261391, Japanese Laid-Open Patent Publication No. 11-185714, and Japanese Laid-Open Patent Publication No. 2001-143664 is applicable even to a small battery, but if the wall thickness is too thinned, the groove may easily rip when the battery is accidentally dropped.
In Japanese Laid-Open Patent Publication No. 2001-256944, the explosion-proof valve can be formed small, but the thin part is formed by press working or cutting. Formation of the thin part by press working is difficult and in the formation by cutting, the manufacturing cost is expensive. Further, when a foreign object hits the thin part, the relevant part is easily ripped and thus handling of the battery in for example, manufacturing steps is troublesome.